1. Field of the Invention
The present invention relates in general to an image obtaining apparatus for irradiating a target subject with an illuminating light and obtaining an optical image based on the re-radiated light re-radiated from said target subject upon the irradiation thereof by said illuminating light, and in particular to an image obtaining apparatus employing a charge multiplying type solid state image obtaining element to obtain images.
2. Description of the Related Art
There are known image obtaining apparatuses, which utilize a solid state image obtaining element such as a CCD or the like for converting an optical image to an electric signal, for obtaining an optical image of a target subject. In recent years, as described, for example, in Japanese Unexamined Patent Publication No. 7(1996)-176721, charge multiplying type solid state image obtaining elements for multiplying an obtained signal charge by a multiplication rate based on a multiplication rate control signal have been developed; wherein, by providing this charge multiplying type solid state image obtaining element, the image obtaining sensitivity of the image obtaining apparatus can be improved and controlled. That is to say, even if the light quantity of an optical image is insufficient to be obtained as an image utilizing a conventional image obtaining apparatus, if said optical image is obtained by use of this charge multiplying type solid state image obtaining element, said optical image can be displayed as a visible image; moreover, the image obtaining sensitivity can be controlled so as to match the image obtaining conditions. A charge multiplying type solid state image obtaining element provided with a charge multiplying means such as that described above is called a CMD (Charge Multiplying Detector) xe2x88x92CCD; wherein, conduction electrons and atoms are made to collide within a high-intensity electric field region, and the charge multiplication effect produced by this ionization serves to multiply the signal charge, whereby the image obtaining sensitivity can be improved.
According to a charge multiplying type solid state image obtaining element, because the charge multiplying means is a means for multiplying the signal charges which is situated in a series of signal processing means in the signal processing sequence as the means before a charge detecting circuit for converting the signal charges into sequential signal voltages and obtaining said voltages as an output signal, the charge multiplying means does not multiply the readout noise produced by the charge detecting circuit, the S/N ratio can be improved thereby. Accordingly, by using a charge multiplying type solid state image obtaining element, it becomes possible to improve the S/N ratio of the output signals of an imaging device that performs image obtainment under conditions in which there is insufficient light for an optical image. Further, because the signal charge multiplication rate can be changed by the multiplication rate control signal, it becomes possible to control the image obtaining sensitivity of an image obtaining apparatus provided with a charge multiplying type solid state image obtaining element.
Further, endoscope apparatuses employing a solid state image obtaining element are in wide use. By displaying the images obtained by a CCD on a monitor or the like, these endoscope apparatuses feature the advantage of being able to allow observation of the image by a plurality of people simultaneously. In addition, by subjecting an obtained image to various image processes before displaying said image, characteristics of the image can be enhanced and the image displayed on a monitor, making a great contribution to the advancement of medicine.
According to these endoscope apparatuses, making use of the fact that the intensity of the fluorescent light emitted from a normal tissue differs from the intensity of the fluorescent light emitted from a diseased tissue when a target subject (i.e., a living tissue) is irradiated by an excitation light having a predetermined wavelength, by detecting the fluorescent light emitted from a target subject upon irradiation thereof by an excitation light having a predetermined wavelength, the location and range of penetration of a diseased tissue is displayed as a fluorescence image, and the tissue state of a diseased portion is determined. However, because there is unevenness on the surface of a target subject, the distance between the light emitting system for emitting the excitation light and the target subject is not uniform; therefore, the intensity of the excitation light irradiating the target subject is generally not of a uniform intensity. Further, although the intensity of the fluorescent-light emitted from the target subject is substantially proportional to the intensity of the excitation light, the intensity of the aforementioned excitation light becomes weaker in inverse proportion to the square of the distance between the excitation light and the target subject. Therefore, there are cases in which the fluorescent light emitted from a diseased tissue located at a position closer to the excitation light source than a normal tissue is of a higher intensity than the fluorescent light emitted from aforementioned normal tissue. Under such conditions, if an observer of an image obtained by the use of the above-described apparatus makes a determination as to the tissue state of the target subject based solely on the data relating to the intensity of the fluorescent light received from the target subject upon the irradiation thereof with an excitation light, there will be cases in which a false determination of the tissue state of the target subject will be made.
In order to mitigate the negative effects of the problems described above, there has been described an image display apparatus in Japanese Unexamined Patent Publication No. 9(1997)-308604, wherein: two types of fluorescence images, e.g., a narrow band fluorescence image having a wavelength near 480 nm, in which the difference in the intensity of the fluorescent light emitted from a normal tissue and the intensity of the fluorescent light emitted from a diseased tissue is large, and a wide band fluorescence image formed of light having wavelengths within the visible spectra of 430-730 nm, for example, are obtained; the ratio of the intensity of the narrow band fluorescence image and the wide band fluorescence image is obtained; and a pseudo color image is displayed based on this ratio. By obtaining the ratio described above, because the factor of the dependency of the intensity of the fluorescent light on the distance between the excitation light source and the fluorescent light receiving portion, and the target subject is cancelled, it is possible to form an image in which only the difference in the spectral form of the fluorescent light is reflected.
On the other hand, the miniaturization of the diameter of the endoscope has seen progress in recent years, and whereas in the past endoscopes were limited to being employed to examine the intestinal tract, currently, endoscopes are employed to examine the respiratory tract, the otorhinolaryngological cavity and passages, and the joints. However, because the number of light guiding fibers for transmitting the illuminating light becomes limited in accordance with the miniaturization of the endoscope, there are cases for which it is not possible to emit sufficient illuminating light; therefore, there is a demand for the development of an image obtaining apparatus capable of obtaining images at a desired image obtaining sensitivity. Further, fluorescence image observation of the fluorescence image obtained of the fluorescent light emitted from a target subject upon the irradiation thereof by an excitation light is also performed. Because the fluorescent light emitted from a target subject upon the irradiation thereof by an excitation light is extremely faint, there are cases in which the obtainment thereof as an image is impossible with current apparatuses; therefore, there is a demand for the development of an image obtaining apparatus capable of obtaining images at a desired image obtaining sensitivity. In order to solve these problems, the structure of and the method of controlling the image obtaining sensitivity for an apparatus wherein the charge multiplying type solid state image obtaining element is installed in an endoscope apparatus have been described in Japanese Unexamined Patent Publication No. 2001-29313
However, the even in an endoscope apparatus employing a charge multiplying type solid state image obtaining element, there are cases in which the fluorescent light emitted from a diseased tissue located at a position closer to the excitation light source than a normal tissue is of a higher intensity than the fluorescent light emitted from aforementioned normal tissue. Therefore, if an observer of an image obtained by the use of the above-described apparatus makes a determination as to the tissue state of the target subject based solely on the intensity of the fluorescent light received from the target subject upon the irradiation thereof with an excitation light, there will be cases in which a false determination of the tissue state of the target subject will be made.
The present invention has been developed in consideration of the circumstances described above, and it is an object of the present invention to provide an image obtaining apparatus such as an endoscope apparatus or the like employing a charge multiplying type solid state image obtaining element that is capable of obtaining images in which the tissue state of a target subject can be accurately discerned.
The image obtaining apparatus according to the present invention comprises: a light emitting means for projecting an illuminating light that includes an excitation light onto a target subject; and a charge multiplying type solid state image obtaining element for obtaining an optical image based on the re-radiated light, which contains fluorescent light, re-radiated from the target subject upon the irradiation thereof by the illuminating light, and obtaining an output data representing the obtained optical image; wherein
the solid state image obtaining element is an element for obtaining fluorescence images based on fluorescent light of mutually different wavelength ranges.
The referent of xe2x80x9cexcitation lightxe2x80x9d includes light having a wavelength in the 400-420 nm wavelength range.
The referents of xe2x80x9cilluminating lightxe2x80x9d include, for example, aside from the excitation light, white light, light in the three primary color wavelength ranges, which are sequentially emitted, and near-infrared light, which is not readily absorbed by the target subject.
Note that the light emitting means can be a means of a configuration employing a simple white light source, wherein a filter that transmits the light in the excitation light wavelength range and other wavelength ranges is used to change the wavelength of the illuminating light projected onto the target subject. Further, the light emitting means can be a means of a configuration employing a light source that emits excitation light, and a light source that emits light in the other wavelength ranges; wherein the wavelength range of the illuminating light projected onto the target subject is sequentially changed by switching between a mode in which the excitation light source is driven and a mode in which the other light source is driven.
The xe2x80x9cre-radiated lightxe2x80x9d refers to the light generated by the target subject upon the irradiation thereof by the illuminating light; more specifically, the xe2x80x9cre-radiated lightxe2x80x9d includes the fluorescent light emitted from a target subject upon the irradiation thereof by the excitation light, the reflected light reflected from the target subject upon the irradiation thereof by the white light, the light in the three primary color wavelength ranges or the near-infrared light, or the scattered light scattered at and emitted from the vicinity of the surface of the target subject.
Therefore, the image obtaining apparatus according to the present invention can obtain a diagnostic fluorescence image based on the fluorescent light emitted from a target subject upon the irradiation thereof by the excitation light, and a reflectance image based on the reflected light reflected from the target subject upon the irradiation thereof by a white light or the like.
The expression xe2x80x9cobtaining fluorescence images based on fluorescent light of mutually different wavelength rangesxe2x80x9d refers to the utilization of an optical means such as a filter or a prism to extract fluorescent light having mutually different wavelength ranges from the fluorescent light emitted from the target subject upon the irradiation thereof by the excitation light, and forming a respective fluorescence image from each of said fluorescent light of mutually different wavelength ranges. For example, a narrow band fluorescence image formed based on the fluorescent light that has passed through a narrow band filter that transmits light having a wavelength near 480 nm, and a wide band fluorescence image formed based on the fluorescent light that has passed through a wide band filter that transmits light having wavelengths in the 400-750 nm wavelength range may be obtained.
Note that according to the image obtaining apparatus of the present invention, it is preferable that a correcting means for detecting the dark noise generated by the solid state image obtaining means, and correcting the output data, based on the detected dark noise, to obtain a corrected output data is provided.
The referent of xe2x80x9cdark noisexe2x80x9d includes not only the dark noise obtained of the solid state image obtaining means, but also, for the case in which the image obtaining apparatus of the present invention is employed in an endoscope apparatus, for example, the noise generated due to the external light transmitted through living tissue.
In this case, the correcting means can be a means for: causing the emission of the illuminating light from the light emitting means on to the target subject to be paused periodically at regular intervals; detecting the dark noise based on the output signal obtained by the solid state image obtaining means during the interval in which the emission of said illuminating light has been paused; and correcting, based on the dark noise, the output data obtained by the solid state image obtaining means during the interval in which the illuminating light has been projected onto the target subject to obtain a corrected output data.
Further, the correcting means can be a means for correcting the output data representing the fluorescence image.
Still further, the image obtaining apparatus according to the present invention may further comprise a rotating filter means having at least two filter elements, each of which transmits a different wavelength range of light; wherein, by the rotation of the different filter elements, each of said filter elements can be made to positionally correspond with the light receiving surface of the solid state image obtaining means.
In addition, according to the image obtaining apparatus of the present invention, the solid state image obtaining means can be provided on the light receiving surface thereof with a filtering means formed of a combination of a plurality of two types of filter elements, each of which transmits a different wavelength range of light, disposed alternately on a two-dimensional flat surface.
Further, according to the image obtaining apparatus of the present invention, a portion or the entirety of the light emitting means and the solid state image obtaining means can be provided in the form of an endoscope for insertion into a body cavity of a patient.
According to the present invention, an image obtaining apparatus comprising a solid state image obtaining element provided with a charge multiplying means obtains fluorescence images based on fluorescent light having mutually different wavelength ranges. In this manner, by obtaining the ratio of the output data representing these fluorescence images, the factor of the distance between the target subject and the re-radiated light can be cancelled, and an image reflecting only the difference between the intensities of the spectra of the fluorescent light can be obtained. Accordingly, the tissue state of the target subject can be accurately discerned.
Here, when the ratio of the output data is to be obtained, particularly if the S/N ratio of the output data, which is the denominator is poor, the ratio is changed by a large amount, and the accuracy of the distinguishability of the tissue state of the target subject is reduced. On the other hand, the noise of the output data obtained by a solid state image obtaining element provided with a charge multiplying means is controlled. Therefore, by detecting the dark noise generated by the solid state image obtaining means, and correcting the output data based on the dark noise, the noise component of the output data can be reduced and the S/N ratio improved thereby. Accordingly, the ratio between the output data representing the fluorescence images can be obtained more accurately; as a result, the distinguishability of the tissue state of the target subject can be improved.
Here, while the emission of the illuminating light from the light emitting means onto the target subject is paused, the output signal obtained by the solid state image obtaining means represents the dark noise of the solid state image obtaining means. Accordingly, by pausing the emission of the illuminating light from the light emitting means onto the target subject and obtaining the output signal of the solid state image obtaining means during the interval in which the emission of the illuminating light is paused, the dark noise of the solid state image obtaining means can be easily detected. Further, by periodically detecting the dark noise of the solid state image obtaining means at regular intervals and correcting the output data obtained by the solid state image obtaining means during the interval in which the illuminating light is being projected onto the target subject, even if the dark noise changes, because correction data reflecting the change in the dark noise can be obtained, the ratio between the output data can be obtained accurately, and the tissue state of the target subject can be discerned more accurately.
In particular, because the intensity of the fluorescent light is extremely weak, if the output data representing the fluorescence images is corrected, fluorescence images having a further improved S/N ratio can be obtained, and as a result, the tissue state of the target subject can be discerned more accurately.
Further, by providing the rotating filter means, because different wavelength ranges of transmitted fluorescent light can be easily projected onto the solid state image obtaining means, the fluorescence images can be easily obtained. Further, because it is easy to provide the rotating filter means with a filter element that transmits wavelengths of light other than that of the excitation light (e.g., light in the three primary color wavelength ranges, near-infrared wavelengths, etc.), fluorescence image based on various wavelength ranges of light can be easily obtained. In this case, because it becomes unnecessary to provide solid state image obtaining means corresponding to various wavelength ranges of light, the configuration of the apparatus can be simplified.
Still further, by providing on the light receiving surface the of the solid state image obtaining means a filtering means formed of a combination of a plurality of two types of filter elements, each of which transmits a different wavelength range of light, disposed alternately on a two-dimensional flat surface, fluorescence images based on a different wavelength ranges of light can be obtained by a use of a simplified configuration; whereby the configuration of the image obtaining apparatus can be simplified. In particular, if the obtainment of fluorescence images formed of light of a plurality of mutually different wavelength ranges is performed based on light that has been transmitted by the wide band filter element, because it becomes unnecessary to provide solid state image obtaining means corresponding to various wavelength ranges of light, the configuration of the apparatus can be further simplified.